The original idea of a spirit level was invented many, many decades ago. Since that time new tools have been developed which utilize a spirit level in conjunction with other tools such as tape measures and lights, described in U.S. Pat. Nos. D349,462 and 2,939,947, respectively.
A common spirit level configuration has two levels placed within a level body. The levels are placed at right angles to one another to allow a user to determine deviations from levelness of a surface which is nominally coplanar with the two spirit level vials. Some (examples of level devices of this nature are U.S. Pat. No. 161,155 and German Patent Nos. 29 04 790 and 60 435. A single vial detects only variations which are perpendicular to the vial, but is unlikely to detect variations which are parallel to the vial. Thereby explaining the principle behind having two vials which are at right angles. This configuration most accurately detects any imperfections.
Another variation of leveling devices uses a rotatable spirit level in order to verify and/or level a surface to angles other than zero. In these cases, the vial is rotatably mounted within the level body. However, the rotation procedure adds another opportunity at which error may be introduced. If a user is careless when rotating the vial or must estimate when the vial has been rotated to the proper angle, the vial may not be at the proper orientation and the leveling procedure will not accurately reflect the state of the surface. Examples of this style of level are shown in U.S. Pat. Nos. 4,590,682 and 4,593,475.